(1) Field of the Invention
The present invention relates to the general technical field of systems for ensuring operating safety of the engines of an aircraft, in general an aircraft having a plurality of engines, and in particular the operating safety of a twin-engined helicopter or rotorcraft.
(2) Description of Related Art
The present invention relates more particularly to protecting a twin-engined power plant in the event of one of the two engines overspeeding.
The engines of an aircraft are generally protected against overspeeding. Such overspeeding may lead to engine speed running away and the engine bursting. It is therefore appropriate to design safety systems that prevent consequences of that kind, since they are catastrophic for the aircraft and its occupants.
Thus, a known safety system relies on the assumption that both engines overspeeding at the same time is very unlikely. Thus, on a card for providing protection against engine overspeed, when one of the engines is detected as running too fast, the engine in question is shut down. Known systems then prevent the second engine that continues to be powered, i.e. the last remaining powered engine on a twin-engined aircraft, from being shut down regardless of its speed. Such prevention is referred to as “cross inhibition”. Such a solution consists in inhibiting the protection against overspeed for the second engine that is still powered. In the event of the second engine overspeeding, after the overspeeding first engine has been shut down, said second engine is no longer authorized to engage its protection against overspeeding.
Under such circumstances, the second engine is no longer protected against overspeeding. Such a situation is improbable but not impossible. Consequently, if new overspeeding conditions occur, the second engine cannot be shut down automatically since its protection is inhibited. Should that occur, there would be a risk of the second engine running away and bursting.
Protection systems are also known that do not involve cross inhibition. Under such circumstances, the second engine is protected against overspeeding, but not against an untimely common mode of both engines overspeeding simultaneously, as might occur during a rotor transient or in a strong electromagnetic field.
The two above-mentioned protection systems thus have their respective advantages and drawbacks. The protection provided by known systems is therefore not optimized.
It is thus possible to consider that an untimely common mode of both engines overspeeding simultaneously is possible. Among potential causes of such a mode, there are for example phenomena such as a rotor transient or a strong electromagnetic field. The rotor transient from which protection is required is that which may be generated, for example, while performing very severe maneuvers (not authorized in the flight manual) concerning very fast drops in collective pitch. Dangerous electromagnetic fields are those generated by very powerful radars that can disturb electrical signals concerning engine speed.
It should be observed that the following documents are known: U.S. Pat. No. 4,500,966; U.S. Pat. No. 5,363,317; and U.S. Pat. No. 3,514,055.
Document U.S. Pat. No. 4,500,966 suggests increasing the power of one engine after failure of another engine.